The present invention relates to a power supply stage of an electric appliance, usable in particular in battery chargers for charging batteries of electric vehicles.
The use is known and common of battery chargers generally integrated inside an electric vehicle, connectable at input to an alternate current power line and connected at output to an electric battery of the electric vehicle.
By way of example, a possible power supply stage A of a battery charger of known type is shown in FIG. 1 and comprises an input filter F, which can be connected to the external power supply line, an ignition current limiter ICL (or also “inrush current limiter”), a rectifier bridge R and a power factor correction circuit PFC.
In particular, in order to comply with the regulations on electrical disturbances, the purpose of the power factor correction circuit PFC is to enable a current to be picked up from the mains which is as sinusoidal as possible.
Always in order to comply with applicable regulations on electrical disturbances, a filter EMC is normally fitted upstream of the PFC itself. The PFC output is always a voltage higher than that of the mains, stabilized with respect to the latter, which is usually used for a further conversion stage.
It is further known that one of the most hazardous events in systems powered by three-phase voltage with neutral, in the event of the three voltages being used as single-phase voltages, is the loss of the neutral connection.
In this situation in fact, if absorptions are unbalanced a virtual star centre is created with potential different from zero.
It therefore follows that the starred voltages can also take on very high values with consequent danger for the integrity of the user devices.
In particular, in the single-phase electronic systems connected to a three-phase power supply, the loss of the neutral can result in the components at supply unit input undergoing voltages in excess of plate values. The simplest way of avoiding the destruction of the device is to monitor the input voltage and interrupt the line in case of overvoltage.
Such monitoring and interruption of the line are normally carried out by an overvoltage protection circuit OP fitted upstream of the filter F.
More specifically, two solutions are known and used.
A first known solution, schematically shown in FIG. 2, envisages the implementation of the (overvoltage) protection circuit OP by means of a switch SW in parallel, controlled by a voltage detector VD, and by an overcurrent breaker BRK arranged in series.
Consequently, following the detection of an overvoltage by means of the voltage detector VD the switch SW is operated and then closed, with consequent interruption of the line by means of the breaker BRK.
This solution, however, is not without drawbacks.
In fact, first of all the reaction speed must be high or in any case compatible with the degree of “withstanding” of the components undergoing the overvoltage. This inevitably calls for the use of a switch SW of the type of an electronic switch.
Furthermore, this solution necessarily requires manual intervention to reset the line and the overcurrent protection and this is not always possible nor “acceptable” to the end user, especially if the protection consists of a fuse and not a resettable thermal magnetic switch.
A second known solution, shown schematically in FIG. 3, envisages the implementation of the protection circuit OP by means of a switch SW in series controlled by a voltage detector VD.
Consequently, following the detection of an overvoltage by means of the voltage detector VD, the switch SW is operated and therefore opened, with consequent line interruption.
This solution too, however, is not without drawbacks.
In fact, in this case as well, a high reaction speed is required and therefore, the use is called for of a switch SW of the type of an electronic switch.
With reference to such second solution, furthermore, we have a high dissipation of the component switch SW which, for reasons of response speed, cannot be a simple relay but normally consists of a semi-conductor electronic switch.